1. Field of the Invention
The invention concerns a composite product composed of a fiber-reinforced material and a further material of the type wherein the magnitude of the coefficient of thermal expansion αc/c of the fiber-reinforced material is direction dependent and depends on the preferred orientation of the fibers. The invention moreover concerns an anode for an x-ray tube in which such a composite material with a bond is used.
2. Description of the Prior Art
Material bonds are used in order to be able to combine mechanical or physical properties of individual materials in a common component (composite product). The individual materials can be of different types, such as, for example, natural materials such as wood, building materials such as cement, or materials such as synthetics (plastics). Composite materials also can be used, for example, with concrete reinforced with steel mesh (known as reinforced concrete), with foils reinforced with textiles, or fiber-reinforced materials, for example glass fiber-reinforced plastic or carbon fiber-reinforced graphite. Fiber-reinforced materials also make use of a combination of the various advantages of the materials of which they are composed. For example, the elongation of a tear of the fiber-reinforced material in the direction of the fiber orientation is increased by a high elongation at the break of the fibers. Analogously, for example, a high elasticity can be achieved. Fiber-reinforced materials combine such advantages of the fibers with the advantages of the other material, for example a lower weight. The physical and mechanical properties of fiber-reinforced materials, such as elongation at a break, elongation at a tear, heat conductivity, expansion factor or electrical conductivity, vary dependent on direction, and depend on the fiber orientation.
Material bonds are used, for example, in aerospace, where extraordinarily resistant and elastic structures are required with, at the same time, the lowest possible weight. They also are used in the construction of buildings and bridges where, with the most cost-effective materials possible, static, highly stressable, and moreover long-term stable constructions are required. To combine various electrical properties, material bonds are used in the production of electrical circuit boards composed of isolators and conductors. A further example is the use of material bonds in anodes for x-ray tubes, in order to achieve a combination of advantageous mechanical properties (for example low weight and high stability) and physical properties (for example high heat conductivity and suitable coefficient of thermal expansion).
Depending on the area of application, material bonds are subject to extraordinarily severe thermal stresses. The problem occurs that the materials in the composite product can exhibit different thermal expansion factor that, most notably, lead to large mechanical tensions between the bonded materials given changing temperatures. The tensions can lead to warping of the component, to tears, chipping, flaking or spalling, or to the separation of connected materials. Such thermal problems can already occur in the production process when these involve changing, possibly very high, temperatures. It can occur, for example, that, in thermally aided coating processes, no layer bonding at all can be achieved between the materials, and thus no material bond is produced.
This problem has an effect in a particularly pronounced manner in anodes for x-ray tubes. These are struck by electrons from the cathode of the x-ray tube and generate x-ray radiation from the kinetic energy of the incident electrons. The anode is thereby significantly heated by the electron bombardment. In order to spread the thermal load on the surface of the anode, rotating anodes are typically used in which a circular focal path on the surface of the anode is used in place of a fixed focal spot to generate the x-ray radiation. The anode is rotated by a shaft that normally is composed of a heat-resistant material, for example molybdenum. An anode plate sits on the shaft that, for example, can likewise be composed of molybdenum, or of graphite, and that has a focal path coating suitable for generation of x-ray radiation. This can be formed, for example, of tungsten or from a tungsten-rhenium alloy. Alternatively, the anode plate can be formed of the same material as the focal path coating, as an integrated component. The use of anode plates made of graphite has the advantage that the heat produced on the focal path coating can be well distributed and removed due to the large heat capacity and heat conductivity of graphite, and can be well radiated away due to its heat radiation properties.
In addition to the thermal stress, x-ray tube anodes are also subjected to a significant mechanical stress. The anode (thus focal path, anode and shaft) typically rotates with a rotation speed of just under 3000/min, which makes a strong, precise and stable positioning of the shaft necessary. In order to keep destabilization of the positioning of the shaft low, light anode plates are advantageous. For this reason graphite exhibits advantages in comparison to metals such as molybdenum or tungsten, due to its lower specific weight. The mechanical load of the rotation positioning multiplies given the use of the x-ray tube in computed tomography (CT), in which the x-ray tube is rotated around the patient with rotation speeds of more than 100/min. Depending on the arrangement of the anode or of the shaft, additional mechanical high-stressing centrifugal forces and corolis forces occur.
Given the increased mechanical stresses as a result of the rotation of the anode itself, as well as the entire anode in the CT, and the increased thermal stress given momentary, extreme increases of the x-ray output, most of all in CT applications, the stability of anode plates made of graphite is increasingly critical. Furthermore, the predominant tendency is toward further increases of the CT rotation speeds. The reduction of the individual image times required as a result of this in turn requires an additional increase of the momentary output, and thus an additional increase of the thermal stress. In the future, materials that are even more stressable than is already the case today will be required.
A material that exhibits a high thermal stressability, a low weight and excellent mechanical properties is carbon fiber-reinforced graphite. This material therefore would be an ideal material for a material bond with, depending on the application, further materials to be selected. In particular, graphite would also be an ideal material for a material bond for application in x-ray anodes. However, due to the different coefficient of thermal expansion, production of a stable material bond between carbon fiber-reinforced graphite and the focal path coating (which includes a transition metal such as tungsten) has not been able to be achieved, much less used. The same is true for many other fiber-reinforced materials that should be bound with additional material, and that have proved to be insufficiently stable either in the production or in the subsequent application. The advantages that would arise from a composite of fiber-reinforced materials with further materials thus far have not been realized.